use strict;
use warnings;
use Getopt::Long;
Getopt::Long::Configure('prefix=+');

my %opt;
GetOptions(\%opt, qw(help offset=i)) or die "error...\n";
$opt{offset} = 3 unless exists $opt{offset};

my $value = shift;

print ' result = ', $value + $opt{offset}, "\n";

##</code><code>##

$ add.pl 5
 result = 8
$ add.pl -5
 result = -2
$ add.pl +offset -2 12
 result = 10